Synthetic polymer that is easily made from petroleum as raw material and has been produced with very large amount on the earth causes social problem for disposal of waste polymer, because it emits poisonous gas when it burns and does not degrade in natural surroundings. Additionally, it is pointed out that petroleum-made plastics, such as polystyrene or polycarbonate, contain environmental hormone which may injure living of the human beings. Additionally plastics often contain some oligomer, but it has been warned that the oligomer may harm the human bodies.
It is also noted to replace petroleum plastics with agricultural product, especially starch material, in view of energy and resource strategy after exhaustion of petroleum resource and zero-emission system of carbon dioxide.
Thus another polymeric material obtained from starch or wood has been developed in lieu of such polymeric materials formed from petroleum resources, because they are harmless to human bodies and do not destroy natural circumstance. These products have been used for years and are safe to human bodies. They are also buried in earth and degraded with bacteria or microorganism.
Some products formed from starch have already been available, such as starch based loose bill obtained by extrusion processing technology in the presence of water, and trays or cups obtained by molding starch slurry with heat and foaming. The starch products are, however, poor in water resistance and strength characteristics, in comparison with those obtained from synthetic polymers. It is also proposed that starch is mixed with other biodegradable synthetic polymers to form into film, sheet or molded article, but the final products do not have sufficient properties as required for industrial products. It is still desired to develop starch products having sufficient physical and chemical properties equal to those obtained from petroleum bas d synthetic polymers.
There are some problems inherent to starch products as follow:                (a) Natural starch is generally composed of a mixture of amylose (polymer having a construction of linearly bonded glucoses) and amylopectin (bunch-shape polymer composed of amylose having branches). The linear amylose has good processability, film properties and molding abilities equal to synthetic plastics, but the amylopectin shows poor strength characteristics. Natural starch, however, does not have so much amylose content. For example, corn starch has amylose content of as low as about 25% and even high-amylose corn starch has an amylose content of about 70% or less.        
Accordingly, it is difficult to employ natural starch as it is, in view of poor processability and moldability.                (b) Amylose can be extracted or separated from natural starch, but its process is complicated and yield by amylose is very low. The process does not have industrial cost effectiveness.        (c) Amylose present in natural starch generally has a molecular weight of as low as about ten thousand Da to hundred thousand Da and therefore is easily retrogradated and shows poor mechanical strength. Accordingly, ever if amylose is effectively extracted or separated from natural starch, the resultant amylose dose not has sufficient characteristics for substituting plastics.        (d) Amylose present in natural starch has a molecular weight distribution (Mw/Mn) of not less than 1.3 and therefore is a mixture of (i) low molecular weight amylose that easily crystallizes, (ii) high molecular weight amylose that easily dissolves in water and (iii) middle molecular weight amylose that is easily gelled. The three type amyloses impede with each other and the mixture does not exhibit enough characteristics in processability or the like.        (e) Amylose present in natural starch does not have complete linear structure, but has small amount of branched structure. Thus, the nucleation speed of natural amylose is fast and the natural amylose easily crystallizes by itself. This characteristics of natural amylose make the structure of film or sheet ununiform and significantly reduces transparency and mechanical strength.        (f) Amylose present in natural starch easily dissolves in hot water of not less than 130° C., but precipitates at a temperature of less than 130° C. (re-crystallization) to form white viscous solution because of the reasons mentioned in the above (c), (d) and (e). The molded article obtained therefrom also has an ununiform structure and shows poor processability, poor transparency and poor strength.        (g) Amylose present in natural starch does not easily dissolve in water at ambient temperature, but dissolves in specific organic solvent, such as dimethylsulfoxide and dimethylformamide. The use of natural amylose requires the process to recover the organic solvent, so that the process is not good for manufacturing because of cost effectiveness. Absence of good and useful solvent is also a serious disadvantage of natural amylose upon altering its polymeric properties by chemical modification.        (h) In order to modify the polymeric characteristics of natural starch, it is also proposed to graft-polymerize starch molecules with vinyl monomer, such as methyl acrylate, methyl methacrylate, or styrene. The modification raises cost of production, but does not enhance polymeric characteristics so much. Additionally the vinyl graft portions do not show biodegradation.        (i) It is difficult for natural amylose to control swelling by chemical crosslinking reaction.        
Industrial application of natural amylose does not proceed so much because of the above mentioned reasons.